GABAergic interneurons have critical roles in signal processing in the cerebral cortex. Moreover, malfunction of these neurons has been implicated in a number of diseases ranging from epilepsy to schizophrenia, anxiety disorders and autism. The goal of Project 2 is to elucidate the functional roles of a group of neocortical GABAergic interneurons that express the ionotropic serotonin (5-HT) receptor 5HT3aR. 5HT3aRexpressing interneurons are the major population of GABAergic neurons in the superficial or associative layers of the cortex. However, largely as a result of the lack of experimental tools to identify and manipulate 5HT3aR interneurons litfie is known about the function of these cells in neocortical signal processing. Based on their location and prevalence, we hypothesize that 5HT3aR interneurons have crucial roles in sensory processing and in long-range interareal cortical communication. Moreover, studies indicate that all 5HT3aR neurons are potently modulated by 5-HT and acetylcholine (ACh) acting on ionotropic 5HT3a and nicotinic receptors, and that they are the main targets of the fast action of these neuromodulators in cortex, as well as of metabotropic responses in some 5HT3aR subpopulations. We hypothesize that these modulations are important in information processing and are involved in shaping cortical circuits during specific brain states and behavioral contexts. These observations have led to the overall hypothesis that 5HT3aR neurons have important roles in context-dependent sensory processing. This project will take advantage of new experimental reagents to begin testing these hypotheses. Electrophysiological recordings combined with optogenetic approaches will be used to study the functional connectivity of 5HT3aR neurons in primary somatosensory cortex with motor cortex and with the higher-order somatosensory thalamic nucleus, the posteromedial nucleus (POm). Photo-stimulation of channel rhodopsin expressing cholinergic and serotonergic axons will be used to study the modulation of 5HT3aR interneuron activity by these subcortical systems, and targeted-patch clamp recordings in-vivo will be used to study the changes in activity of 5HT3aR interneurons during different brain states and in response to sensory simulation and motor activity